Time-of-flight is a method used to measure the time it takes for a signal to travel from a transmitter to a receiver over a distance. The signal used with this method may be acoustic or electromagnetic energy; electromagnetic energy such as x-ray, radio frequency, microwave, millimeter-wave, radar, and laser. Time-of-flight can be used to measure distance, the velocity of a moving object or fluid, or the velocity of a signal over a known distance. It is use in devices such as ultrasonic flow meters, ranging equipments, radar, and navigation systems.
In prior art there are two fundamental methods for measuring time-of-flight; measuring the propagation time of a signal from a transmitter to a receiver (transit time method), and measuring the phase difference between two or more received signals (phase based method).
Ranging devices measure the time-of-flight of a signal from a transmitter to a target and back to a receiver. The time-of-flight and the velocity of the signal through a medium is used to determine the distance to the target.
Ultrasonic transit-time flow meters in prior art measure the time-of-flight of an acoustic pulse transmitted in both direction of fluid flow, and use the difference in the transit time to determine the fluid flow rate. The shape and response time of the pulse is crucial to measuring the time-of-flight with some degree of accuracy. To get good signal to noise ratio a response time between 50-100 pico-seconds and powerful precisely tuned transducers are required; making it relatively expensive. Prior art transit-time devices based on the speed of light require long averaging time and sub-nanosecond timing circuitry to measure distances with good resolution.
Some phase based method use a phase comparator to generate a voltage that is proportional to the phase and use a digital to analog converter to digitize the voltage to determine the phase. Other devices digitize the signal and perform a Fourier transform on the digitized data to determine the phase. The low resolution of analog to digital converters require long averaging time to improve accuracy, and Fourier transform requires intensive computation and increased power usage.